Method of working pc steel bars while subheated



1967 KATUHIISA MIZUMA 3,306,736

METHOD OF WORKING PC STEEL BARS WHILE SUBHEATED Filed April 5, 1964 F 19. E (J) (1 /Loaen ava/n7! W (2) WM WM;

United States Patent 3,306,786 METHOD OF WORKING PC STEEL BARS WHILE SUBHEATED Katuhisa Mizuma, Fujisawa, Japan, assignor to Koshuha Netsuren Kabushiki Kaisha, Tokyo, Japan, a corporationn of Japan Filed Apr. 3, 1964, Ser. No. 357,041 5 Claims. (Cl. 14812.4)

The present invention relates to a method for working steel bars while only the external surface portion thereof is heated, and more particularly to a method for working steel bars of the type used in prestressed concrete structures.

More specifically, the invention relates to a method of forming coupling surfaces such as screw threads on the end portions of the bars by rolling the threads while the surface portion of the bar is heated, the internal por tion of the bar remaining at a lower temperature so that its hardness is not materially affected. Other operations, such as heading, may similarly be performed on the softened surface portion of the bar instead of rolling threads thereon.

The steel to be used usually for PC steel bars is such steel as of 0.30 to 0.70% carbon. Such steel is hardened and tempered and is usually rolled by being cold-worked at both ends. In the method of the present invention, there can be obtained products having performances far superior to those of conventional products by special treatments and working under special conditions.

In the method of the present invention, a reinforcing bar having 0.30 to 0.50% carbon is first high frequencyhardened. If it is hardened by a conventional method wherein it is heated in a furnace and is quenched in oil or water, such fine gauge long material as a steel bar for prestressed concrete will be difficult to manipulate and a uniform hardening effect will be very difiicult to obtain.

On the other hand, using the method of the present invention, the material, throughout its length, is continuously quickly heated and quenched by a conventional high frequency-hardening process so that a uniform hardness is obtained. Further, due to quick heating by high frequencies and very quick quenching the grain structure of the material will not become coarse and its properties will not deteriorate. Thus, there will be obtained a material which is very compact and has favourable prop erties and a very high hardness.

Then, the entire length of the thus hardened material is tempered preferably at a temperature of about 400 C. Thus, though the hardness will become somewhat reduced, the material will have a proper elongation and will be tough.

Due to its non-critical nature, the tempering treatment is possible not only by high frequency-heating but also by any other ordinary method of heating.

By such high frequency-heating and subsequent tempering treatments, the steel of 0.30 to 0.50% carbon will In the method of the present invention, the end portions of the steel bar are heated to the tempering temperature of about 400 C. or higher by high frequency-heating in such a manner that only the surface layer part to be deformed will be heated to the required softening temperature and the inner part may be prevented from being at a high temperature as much as possible.

As a means of heating it so that only the surface layer part to be worked will be heated as much as possible but the inner part will not be at such a high temperature as of the surface layer part, a high frequency-heating process is utilized in the method of the present invention.

In this high frequency-heating which differs significantly from the ordinary furnace-heating, the temperature gradient between the surface layer part and the inner part will be so large that only the part to be worked will be kept substantially at the tempering temperature of about 400 C. but the inner part will notbe substantially influenced by the temperature during the working operation.

Thus, according to the present invention, a steel bar having about 0.30 to 0.50% carbon is first high frequencyhardened, is then' tempered at about 400 C. and is threadedly rolled at its ends or otherwise worked while surface heated to the tempering temperature of about 400 C. or a little higher by high frequency-heating only the end portions to be threadedly rolled for providing tensioning connections when the bar is in use.

For the tempering, ordinary furnace-heating, bathheating or any other heating process may be used.

In the final heating for thread rolling, if ordinary heating is used, the temperature difference between the surface layer part and the inner part will be so small that even the inner part will be influenced by the final heating and the properties or specifically the mechanical properties of the product will deteriorate. On the other hand, in the present invention, as high frequency-heating is used, only the outer surface layer part required to be worked will be heated to about 400 C. as required and the inner part will be also heated but will be only at a temperature far lower than in the surface layer part. Therefore, particularly in the fatigue resistance required for the prestressed concrete reinforcing bar, there will be shown performances far superior to those of conventional products.

The invention will be better understood upon reading the following specification with reference to the accompanying drawing forming a part hereof.

In the drawing:

FIGURE 1 is a diagrammatic illustration of an apparatus for performing the method of the present invention.

FIGURE 2 is a graph showing the results of fatigue tests of products worked by the method of the present invention and by the conventional method.

An apparatus for performing the method of the present invention is shown in FIGURE 1. In the drawing, 1 is a high frequency electrical generator, 2 is an output transformer, 3 is an induction heating coil, 4 is a screw rolling head, 5 is a rotary feeding device and 6 is a steel bar. First of all, the hardened and tempered steel bar 6 is placed in the rotary feeding device 5 and is fed into the heating coil 3 by means of the feeding device. The power and feeding velocity are so adjusted that the steel bar 6 will become heated as required while it passes through the coil 3. The end portion of the surface heated steel bar is fed as heated into the rolling head 4 and is, for example, screw-rolled.

An example of working the method of the present invention shall be given. A steel bar of 0.30 to 0.50% carbon and a diameter of 14.9 mm. was high frequencyhardened, was then tempered by being kept at 400 C. for 2 hours and Was then threadedly rolled while surface heated by means of the apparatus in FIGURE 1.

The mechanical properties of the previously high frequency-hardened and tempered steel bar were as shown in Table 1.

TABLE 1 Kind: Material to be rolled Tensile strength in kg./mm. 111.2

Yield point in kg./mm. 98.2 Elongation in percent Contraction of area in percent 51 The high frequency-hardened and tempered steel bar was rolled by means of the apparatus in FIGURE 1 under the conditions of a surface temperature of about 400 C., an axial rolling velocity (feeding velocity for the steel bar) of 7.1 mm./sec., a passing time through the heating coil of 4.07 seconds and a distance between the heating coil and the rolling dies of 60 mm.

The comparison of the mechanical properties of the steel bar having had the screw part rolled while surface heated by the method of the invention with those of a steel bar having had the screw part cold-rolled by the conventional method is shown in Table 2.

4 fications may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. The method of forming a prestressable reinforcing bar for use in a concrete structure which comprises the steps of: hardening a steel bar having a carbon content of about 0.30% to 0.50%; tempering the hardened bar at a temperature of about 400 C.; surface heating end portions of said bar to temperature of at least 400 C. while maintaining the end portions located inwardly of said surface heated portions at a temperature materially lower than 400 C. whereby said inwardly located portions remain effectively unchanged in their mechanical properties by said surface heating; and working said surface heated portions while surface heated to form coupling surfaces for prestressing said bar.

2. The method of forming a bar having coupling surfaces at its ends which comprises the steps of: uniformly hardening a steel bar having a carbon content of about 0.30% to 0.50%; uniformly tempering the hardened bar throughout its length; generating an electrical current of high frequency; energizing a coil with said current; passing an end portion of said bar through the field of said coil to heat the external surface portions thereof while maintaining the portions located inwardly of said surface portions at a temperature materially lower than said surface portions for preventing the mechanical properties of said inwardly located portions from being materially affected TABLE 2 Test PC Nos. Kind Perfonnances Mean Conventional method Tensile strength in kg,/rnm 104. 0 101. 2 101. 2 104. 0 104. 5 103. 0

Broken part 0) (0 Method of the present invention Tensile strength in kg./m m 113.8 111. 7 112.6 109. 5 110.5 111. 6

Broken part 1 Screw part.

2 Parallel part.

As is evident from Table 2, the products obtained by the method of the present invention were about 8% higher in tensile strength and failure occurred in the parallel part, whereas the products obtained by the conventional method all failed in the screw part without exception. The bars produced by the method of the present invention were far superior in the strength of the rolled part than those produced by the conventional method.

The results of fatigue tests of the products by the method of the present invention and those by the conventional method are shown in FIGURE 2. These data are by Rosenhousens tester and show that, when the mean stress was 56 kg./mm. the stress amplitude was 8.5 kg./m m. for bars produced by the conventional method, whereas it was 11 kg./mm. for bars produced by the method of the present invention, showing an improvement of about Further, in comparing them for precision of the rolled screw, it was found that they were little different from each other.

While I have shown and described what I believe to be the best embodiment of my invention, it will be apparent to those skilled in the art that various changes and modi- References Cited by the Examiner FOREIGN PATENTS 3/1957 Great Britain. 4/1957 Great Britain.

DAVID L. RECK, Primary Examiner.

H. F. SAITO Assistant Examiner. 

1. THE METHOD OF FORMING A PRESTRESSABLE REINFORCING BAR FOR USE IN A CONCRETE STRUCTURE WHICH COMPRISES THE STEP OF: HARDENING A STEEL BAR HAVING A CARBON CONTENT OF ABOUT 0.30% TO 0.50%; TEMPERING THE HARDENED BAR AT A TEMPERATURE OF ABOUT 400*C.; SURFACE HEATING END PORTIONS OF SAID BAR TO TEMPERATURE OF AT LEAST 400*C. WHILE MAINTAINING THE END PORTIONS LOCATED INWARDLY OF SAID SURFACE HEATED PORTIONS AT A TEMPERATURE MATERIALLY LOW- 